Short answer:
blue is dominant to black.
black is dominant to lavender.
lavender covers blue. But lavender is recessive, see below.
black is dominant to white unless you have the so called dom.white
white covers every other color. But white is recessive, see below.

Long answer:
It depends.
If you cross lavender and white neither is dominant, the crosslings could be black for instance.
If you have a bird that is both lavender and white it will look white. White is the absence of color.
White is epistatic to (="covers") all colors.
If you have a bird that is both blue and chocolate the color will be a combination of both but also more diluted (the dilution adds up)
Lavender and blue do not add up. Lavender is epistatic to most black diluters unless they would be lighter than lavender (eg splash or dom.white).

In genetics, dominance applies to the forms of one gene eg blue (symbol Bl) and not-blue (bl+).
bl+ is the not-mutated form of the blue gene. Present in black chickens for instance.
It is written in lowercase to show that it is recessive (not dominant) and has a plus sign to say that it is wildtype/not mutated.
Bl is the mutation that made us aware that this gene exists and now has a mutated form.
It is written in Uppercase to show that it behaves dominant to the original gene.
Blue is a special case of dominance called incomplete dominance because 2 doses of blue would make an even more diluted color called splash.